Nonvolatile logic array with retention flip flops to reduce switching power during wakeup

ABSTRACT

A processing device is operated using a plurality of volatile storage elements. Data in the plurality of volatile storage elements is stored in a plurality of non-volatile logic element arrays. A primary logic circuit portion of individual ones of the plurality of volatile storage elements is powered by a first power domain, and a slave stage circuit portion of individual ones of the plurality of volatile storage elements is powered by a second power domain. During a write back of data from the plurality of non-volatile logic element arrays to the plurality of volatile storage elements, the first power domain is powered down and the second power domain is maintained. In a further approach, the plurality of non-volatile logic element arrays is powered by a third power domain, which is powered down during regular operation of the processing device.

RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional application No.61/698,906, filed Sep. 10, 2012, which is incorporated by reference inits entirety herein.

TECHNICAL FIELD

This invention generally relates to nonvolatile memory cells and theiruse in a system, and in particular, in combination with logic arrays toprovide nonvolatile logic modules.

BACKGROUND

Many portable electronic devices such as cellular phones, digitalcameras/camcorders, personal digital assistants, laptop computers andvideo games operate on batteries. During periods of inactivity thedevice may not perform processing operations and may be placed in apower-down or standby power mode to conserve power. Power provided to aportion of the logic within the electronic device may be turned off in alow power standby power mode. However, presence of leakage currentduring the standby power mode represents a challenge for designingportable, battery operated devices. Data retention circuits such asflip-flops and/or latches within the device may be used to store stateinformation for later use prior to the device entering the standby powermode. The data retention latch, which may also be referred to as ashadow latch or a balloon latch, is typically powered by a separate‘always on’ power supply.

A known technique for reducing leakage current during periods ofinactivity utilizes multi-threshold CMOS (MTCMOS) technology toimplement the shadow latch. In this approach, the shadow latch utilizesthick gate oxide transistors and/or high threshold voltage (V_(t))transistors to reduce the leakage current in standby power mode. Theshadow latch is typically detached from the rest of the circuit duringnormal operation (e.g., during an active power mode) to maintain systemperformance. To retain data in a ‘master-slave’ flip-flop topology, athird latch, e.g., the shadow latch, may be added to the master latchand the slave latch for the data retention. In other cases, the slavelatch may be configured to operate as the retention latch during lowpower operation. However, some power is still required to retain thesaved state. For example, see U.S. Pat. No. 7,639,056, “Ultra Low AreaOverhead Retention Flip-Flop for Power-Down Applications”, which isincorporated by reference herein.

System on Chip (SoC) is a concept that has been around for a long time;the basic approach is to integrate more and more functionality into agiven device. This integration can take the form of either hardware orsolution software. Performance gains are traditionally achieved byincreased clock rates and more advanced process nodes. Many SoC designspair a microprocessor core, or multiple cores, with various peripheraldevices and memory circuits.

Energy harvesting, also known as power harvesting or energy scavenging,is the process by which energy is derived from external sources,captured, and stored for small, wireless autonomous devices, such asthose used in wearable electronics and wireless sensor networks.Harvested energy may be derived from various sources, such as: solarpower, thermal energy, wind energy, salinity gradients, and kineticenergy, etc. However, typical energy harvesters provide a very smallamount of power for low-energy electronics. The energy source for energyharvesters is present as ambient background and is available for use.For example, temperature gradients exist from the operation of acombustion engine, and in urban areas, there is a large amount ofelectromagnetic energy in the environment because of radio andtelevision broadcasting, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a portion of an example systemon chip (SoC) as configured in accordance with various embodiments ofthe invention;

FIG. 2 is a more detailed block diagram of one flip-flop cloud used inthe SoC of FIG. 1;

FIG. 3 is a plot illustrating polarization hysteresis exhibited by aferroelectric capacitor;

FIGS. 4-7 are schematic and timing diagrams illustrating an exampleferroelectric nonvolatile bit cell as configured in accordance withvarious embodiments of the invention;

FIGS. 8-9 are schematic and timing diagrams illustrating another exampleferroelectric nonvolatile bit cell as configured in accordance withvarious embodiments of the invention;

FIG. 10 is a block diagram illustrating an example NVL array used in theSoC of FIG. 1;

FIGS. 11A and 11B are more detailed schematics of input/output circuitsused in the NVL array of FIG. 10;

FIG. 12A is a timing diagram illustrating an example offset voltage testduring a read cycle as configured in accordance with various embodimentsof the invention;

FIG. 12B illustrates a histogram generated during an example sweep ofoffset voltage as configured in accordance with various embodiments ofthe invention;

FIG. 13 is a schematic illustrating parity generation in the NVL arrayof FIG. 10;

FIG. 14 is a block diagram illustrating example power domains within anNVL array as configured in accordance with various embodiments of theinvention;

FIG. 15 is a schematic of an example level converter for use in the NVLarray as configured in accordance with various embodiments of theinvention;

FIG. 16 is a timing diagram illustrating an example operation of levelshifting using a sense amp within a ferroelectric bitcell as configuredin accordance with various embodiments of the invention;

FIG. 17 is a block diagram of an example power detection arrangement asconfigured in accordance with various embodiments of the invention;

FIG. 18 is a flow chart illustrating an example operation of powering aprocessing device as configured in accordance with various embodimentsof the invention; and

FIG. 19 is a flow chart illustrating another example operation ofpowering a processing device as configured in accordance with variousembodiments of the invention; and

FIG. 20 is a block diagram of another example SoC that includes NVLarrays as configured in accordance with various embodiments of theinvention.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions and/or relative positioningof some of the elements in the figures may be exaggerated relative toother elements to help to improve understanding of various embodimentsof the present invention. Also, common but well-understood elements thatare useful or necessary in a commercially feasible embodiment are oftennot depicted in order to facilitate a less obstructed view of thesevarious embodiments. It will further be appreciated that certain actionsand/or steps may be described or depicted in a particular order ofoccurrence while those skilled in the art will understand that suchspecificity with respect to sequence is not actually required. It willalso be understood that the terms and expressions used herein have theordinary technical meaning as is accorded to such terms and expressionsby persons skilled in the technical field as set forth above exceptwhere different specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

Specific embodiments of the invention will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency. In thefollowing detailed description, numerous specific details are set forthto provide a more thorough understanding. However, it will be apparentto one of ordinary skill in the art that aspects of the invention may bepracticed without these specific details. In other instances, well-knownfeatures have not been described in detail to avoid unnecessarilycomplicating the description.

While prior art systems made use of retention latches to retain thestate of flip-flops in logic modules during low power operation, somepower is still required to retain state. In contrast, nonvolatileelements can retain the state of flip flops in logic module while poweris completely removed. Such logic elements will be referred to herein asNon-Volatile Logic (NVL). A micro-control unit (MCU) implemented withNVL within an SoC (system on a chip) may have the ability to stop, powerdown, and power up with no loss in functionality. A system reset/rebootis not required to resume operation after power has been completelyremoved. This capability is ideal for emerging energy harvestingapplications, such as Near Field Communication (NFC), radio frequencyidentification (RFID) applications, and embedded control and monitoringsystems, for example, where the time and power cost of the reset/rebootprocess can consume much of the available energy, leaving little or noenergy for useful computation, sensing, or control functions. Thoughthis description discusses an SOC containing a programmable MCU forsequencing the SOC state machines, one of ordinary skill in the art cansee that NVL can be applied to state machines hard coded into ordinarylogic gates or ROM, PLA, or PLD based control systems.

In one approach, an SoC includes one or more blocks of nonvolatilelogic. For example, a non-volatile logic (NVL) based SoC may back up itsworking state (all flip-flops) upon receiving a power interrupt, havezero leakage in sleep mode, and need less than 400 ns to restore thesystem state upon power-up.

Without NVL, a chip would either have to keep all flip-flops powered inat least a low power retention state that requires a continual powersource even in standby mode or waste energy and time rebooting afterpower-up. For energy harvesting applications, NVL is useful becausethere is no constant power source required to preserve the state offlip-flops (FFs), and even when the intermittent power source isavailable, boot-up code alone may consume all the harvested energy. Forhandheld devices with limited cooling and battery capacity, zero-leakageIC's (integrated circuits) with “instant-on” capability are ideal.

Ferroelectric random access memory (FRAM) is a non-volatile memorytechnology with similar behavior to DRAM (dynamic random access memory).Each individual bit can be accessed, but unlike EEPROM (electricallyerasable programmable read only memory) or Flash, FRAM does not requirea special sequence to write data nor does it require a charge pump toachieve required higher programming voltages. Each ferroelectric memorycell contains one or more ferroelectric capacitors (FeCap). Individualferroelectric capacitors may be used as non-volatile elements in the NVLcircuits described herein.

FIG. 1 is a functional block diagram illustrating a portion of acomputing device, in this case, an example system on chip (SoC) 100providing non-volatile logic based computing features. While the termSoC is used herein to refer to an integrated circuit that contains oneor more system elements, the teachings of this disclosure can be appliedto various types of integrated circuits that contain functional logicmodules such as latches, integrated clock gating cells, and flip-flopcircuit elements (FF) that provide non-volatile state retention.Embedding non-volatile storage elements outside the controlledenvironment of a large array presents reliability and fabricationchallenges. An NVL bitcell based NVL array is typically designed formaximum read signal margin and in-situ margin testability as is neededfor any NV-memory technology. However, adding testability features toindividual NVL FFs may be prohibitive in terms of area overhead.

To amortize the test feature costs and improve manufacturability, andwith reference to the example of FIGS. 1 and 2, a plurality ofnon-volatile logic element arrays or NVL arrays 110 are disposed with aplurality of volatile storage elements 220. At least one non-volatilelogic controller 106 configured to control the plurality of NVL arrays110 to store a machine state represented by the plurality of volatilestorage elements 220 and to read out a stored machine state from theplurality of NVL arrays 110 to the plurality of volatile storageelements 220. For instance, the at least one non-volatile logiccontroller 106 is configured to generate a control sequence for savingthe machine state to or retrieving the machine state from the pluralityof NVL arrays 110. A multiplexer 212 is connected to variably connectindividual ones of the volatile storage elements 220 to one or morecorresponding individual ones of the NVL arrays 110.

In the illustrated example, the computing device apparatus is arrangedon a single chip, here an SoC 100 implemented using 256b mini-arrays110, which will be referred to herein as NVL arrays, of FeCap(ferroelectric capacitor) based bitcells dispersed throughout the logiccloud to save state of the various flip flops 120 when power is removed.Each cloud 102-104 of FFs 120 includes an associated NVL array 110. Suchdispersal results in individual ones of the NVL arrays 110 beingarranged physically closely to and connected to receive data fromcorresponding individual ones of the volatile storage elements 220. Acentral NVL controller 106 controls all the arrays and theircommunication with FFs 120. While three FF clouds 102-104 areillustrated here, SoC 100 may have additional, or fewer, FF clouds allcontrolled by NVL controller 106. The SOC 100 can be partitioned intomore than one NVL domain in which there is a dedicated NVL controllerfor managing the NVL arrays 110 and FFs 120 in each of the separate NVLdomains. The existing NVL array embodiment uses 256 bit mini-arrays, butthe arrays may have a greater or lesser number of bits as needed.

SoC 100 is implemented using modified retention flip flops 120 includingcircuitry configured to enable write back of data from individual onesof the plurality of non-volatile logic element arrays to the individualones of the plurality of flip flop circuits. There are various knownways to implement a retention flip flop. For example, a data input maybe latched by a first latch. A second latch coupled to the first latchmay receive the data input for retention while the first latch isinoperative in a standby power mode. The first latch receives power froma first power line that is switched off during the standby power mode.The second latch receives power from a second power line that remains onduring the standby mode. A controller receives a clock input and aretention signal and provides a clock output to the first latch and thesecond latch. A change in the retention signal is indicative of atransition to the standby power mode. The controller continues to holdthe clock output at a predefined voltage level and the second latchcontinues to receive power from the second power line in the standbypower mode, thereby retaining the data input Such a retention latch isdescribed in more detail in U.S. Pat. No. 7,639,056, “Ultra Low AreaOverhead Retention Flip-Flop for Power-Down Applications”.

FIG. 2 illustrates an example retention flop architecture that does notrequire that the clock be held in a particular state during retention.In such a “clock free” NVL flop design, the clock value is a “don'tcare” during retention.

In SoC 100, modified retention FFs 120 include simple input and controlmodifications to allow the state of each FF to be saved in an associatedFeCap bit cell in NVL array 110, for example, when the system is beingtransitioned to a power off state. When the system is restored, then thesaved state is transferred from NVL array 110 back to each FF 120. Powersavings and data integrity can be improved through implementation ofparticular power configurations. In one such approach, individualretention flip flop circuits include a primary logic circuit portion(master stage or latch) powered by a first power domain (such as VDDL inthe below described example) and a slave stage circuit portion poweredby a second power domain (such as VDDR in the below described example).In this approach, the first power domain is configured to be powereddown and the second power domain is active during write back of datafrom the plurality of NVL arrays to the plurality of volatile storageelements. The plurality of non-volatile logic elements are configured tobe powered by a third power domain (such as VDDN in the below describedexample) that is configured to be powered down during regular operationof the computing device apparatus.

With this configuration, a plurality of power domains can be implementedthat are independently powered up or powered down in a manner that canbe specifically designed to fit a given implementation. Thus, in anotheraspect, the computing apparatus includes a first power domain configuredto supply power to switched logic elements of the computing deviceapparatus and a second power domain configured to supply power to logicelements configured to control signals for storing data to or readingdata from the plurality of non-volatile logic element arrays. Where theplurality of volatile storage elements comprise retention flip flops,the second power domain is configured to provide power to a slave stageof individual ones of the retention flip flops. A third power domainsupplies power for the plurality of non-volatile logic element arrays.In addition to the power domains, NVL arrays can be defined as domainsrelating to particular functions. For example, a first set of at leastone of the plurality of non-volatile logic element arrays can beassociated with a first function of the computing device apparatus and asecond set of at least one of the plurality of non-volatile logicelement arrays can be associated with a second function of the computingdevice apparatus. Operation of the first set of at least one of theplurality of non-volatile logic element arrays is independent ofoperation of the second set of at least one of the plurality ofnon-volatile logic element arrays. So configured, flexibility in thecontrol and handling of the separate NVL array domains or sets allowsmore granulated control of the computing device's overall function.

This more specific control can be applied to the power domains as well.In one example, the first power domain is divided into a first portionconfigured to supply power to switched logic elements associated withthe first function and a second portion configured to supply power toswitched logic elements associated with the second function. The firstportion and the second portion of the first power domain areindividually configured to be powered up or down independently of otherportions of the first power domain. Similarly, the third power domaincan be divided into a first portion configured to supply power tonon-volatile logic element arrays associated with the first function anda second portion configured to supply power to non-volatile logicelement arrays associated with the second function. As with the firstpower domain, the first portion and the second portion of the thirdpower domain are individually configured to be powered up or downindependently of other portions of the third power domain.

So configured, if individual functions are not used for a given device,flip flops and NVL arrays associated with the unused functions can berespectively powered down and operated separately from the other flipflops and NVL arrays. Such flexibility in power and operation managementallows one to tailor the functionality of a computing device withrespect to power usage and function. This can be further illustrated inthe following example design having a CPU, three SPI interfaces, threeUART interfaces, three I2C interfaces, and only one logic power domain(VDDL). The logic power domain is distinguished from the retention orNVL power domains (VDDR and VDDN respectively), although these teachingscan be applied to those power domains as well. Although this exampledevice has only one logic power domain, a given application for thedevice might only use one of the three SPI units, one of the three UARTsand one of the three I2C peripherals. To allow applications to optimizethe NVL application wake-up and sleep times and energy costs, the VDDLpower domain can be partitioned into 10 separate NVL domains (one CPU,three SPI, three UART, three I2C totalling 10 NVL domains), each ofwhich can be enabled/disabled independently of the others. So, thecustomer could enable NVL capability for the CPU, one SPI, one UART, andone I2C for their specific application while disabling the others. Inaddition, this partitioning also allows flexibility in time as well asenergy and the different NVL domains can save and restore state atdifferent points in time.

To add further flexibility, NVL domains can overlap with power domains.Referring to the above example, four power domains can be defined: oneeach for CPU, SPI, UART, and I2C (each peripheral power domain has threefunctional units) while defining three NVL domains within eachperipheral domain and one for the CPU (total of 10 NVL domains again).In this case, individual power domains turn on or off in addition tocontrolling the NVL domains inside each power domain for addedflexibility in power savings and wakeup/sleep timing.

Moreover, individual ones of the first power domain, the second powerdomain, and the third power domain are configured to be powered down orup independently of other ones of the first power domain, the secondpower domain, and the third power domain. For instance, integral powergates can be configured to be controlled to power down the individualones of the first power domain, the second power domain, and the thirdpower domain. As described in table 1 below, the third power domain isconfigured to be powered down during regular operation of the computingdevice apparatus, and the second power domain is configured to bepowered down during a write back of data from the plurality ofnon-volatile logic element arrays to the plurality of volatile storageelements. A fourth power domain can be configured to supply power toreal time clocks and wake-up interrupt logic.

Such approaches can be further understood in reference to theillustrated example SoC 100 where NVL arrays 110 and controller 106 areoperated on an NVL power domain referred to as VDDN and are switched offduring regular operation. All logic, memory blocks 107 such as ROM (readonly memory) and SRAM (static random access memory), and master stage ofFFs are on a logic power domain referred to as VDDL. FRAM (ferroelectricrandom access memory) arrays are directly connected to a dedicatedglobal supply rail (VDDZ) maintained at a higher fixed voltage neededfor FRAM (i.e., VDDL<=VDDZ, where VDDZ is a fixed supply and VDDL can bevaried as long as VDDL remains at a lower potential than VDDZ). Notethat FRAM arrays as shown in 103 typically contain integrated powerswitches that allow the FRAM arrays to be powered down as needed, thoughit can easily be seen that FRAM arrays without internal power switchescan be utilized in conjunction with power switches that are external tothe FRAM array. The slave stages of retention FFs are on a retentionpower domain referred to as the VDDR domain to enable regular retentionin a stand-by mode of operation. Table 1 summarizes power domainoperation during normal operation, system backup to NVL arrays, sleepmode, system restoration from NVL arrays, and back to normal operation.Table 1 also specifies domains used during a standby idle mode that maybe initiated under control of system software in order to enter areduced power state using the volatile retention function of theretention flip flops. A set of switches indicated at 108 are used tocontrol the various power domains. There may be multiple switches 108that may be distributed throughout SoC 100 and controlled by softwareexecuted by a processor on SoC 100 and/or by a hardware controller (notshown) within SoC 100. There may be additional domains in addition tothe three illustrated here, as will be described later.

TABLE 1 system power modes Trigger SoC Mode Trigger source VDDL VDDRVDDN Regular operation na na ON ON OFF System backup to Power externalON ON ON NVL bad Sleep mode Backup NVL OFF OFF OFF done controllerSystem restoration Power external OFF ON ON from NVL good Regularoperation Restore NVL ON ON OFF done controller Standby retention idleSystem OFF ON OFF mode software

State info could be saved in a large centralized FRAM array, but wouldrequire a more time to enter sleep mode, longer wakeup time, excessiverouting, and power costs caused by the lack of parallel access to systemFFs.

FIG. 2 is a more detailed block diagram of one FF cloud 102 used in SoC100. In this embodiment, each FF cloud includes up to 248 flip flops andeach NVL array is organized as an 8×32 bit array, but one bit is usedfor parity in this embodiment. However, in other embodiments, the numberof flip flops and the organization of the NVL array may have a differentconfiguration, such as 4×m, 16×m, etc, where m is chosen to match thesize of the FF cloud. In some embodiments, all of the NVL arrays in thevarious clouds may be the same size, while in other approaches there maybe different size NVL arrays in the same SoC.

Block 220 is a more detailed schematic of each retention FF 120. Severalof the signals have an inverted version indicated by suffix “B”(referring to “bar” or /), such as RET and RETB, CLK and CLKB, etc. Eachretention FF includes a master latch 221 and a slave latch 222. Slavelatch 222 is formed by inverter 223 and inverter 224. Inverter 224includes a set of transistors controlled by the retention signal (RET,RETB) that are used to retain the FF state during low power sleepperiods, during which power domain VDDR remains on while power domainVDDL is turned off, as described above and in Table 1.

NVL array 110 is logically connected with the 248 FFs it serves in cloud102. Generally speaking, to enable data transfer from an NVL array tothe FFs, individual FFs include circuitry configured to enable writeback of data from individual ones of the plurality of NVL arrays 110. Inthe illustrated example, two additional ports are provided on the slavelatch 222 of each FF as shown in block 220. A data input port (gate 225)is configured to insert data ND from one of the NVL arrays 110 to anassociated volatile storage element 220. The data input port isconfigured to insert the data ND by allowing passage of a stored datarelated signal from the one of the NVL arrays to a slave stage of theassociated flip flop circuit in response to receiving an update signalNU from the at least one non-volatile logic controller 106 on a datainput enable port to trigger the data input port. Inverter 223 isconfigured to be disabled in response to receiving the inverted NVLupdate signal NUZ to avoid an electrical conflict between the tri-stateinverter 223 and the NVL data port input tri-state inverter 225.

More specifically, in the illustrated example, the inv-inv feedback pair(223 and 224) form the latch itself. These inverters make a very stableconfiguration for holding the data state and will fight any attempts tochange the latch state unless at least one of the inverters is disabledto prevent electrical conflict when trying to overwrite the currentstate with the next state via one of the data ports. The illustrated NVLFF 220 includes two data ports that access the slave latch 222 ascompared to one data port for a regular flop. One port transfers datafrom the master stage 221 to the slave stage 222 via the cmos pass gatecontrolled by the clock. When using this port to update the slave state221, the inverter 224 driving onto the output node of the pass gatecontrolled by CLK is disabled to avoid an electrical conflict while theinverter 223 is enabled to transfer the next state onto the oppositeside of the latch so that both sides of the latch have the next state inpreparation for holding the data when clock goes low (for a posedge FF).

For the same reason, the inverter 223 is disabled when the ND data portis activated by NU transitioning to the active high state to avoid anelectrical conflict on the ND port. The second inverter 224 is enabledto transfer the next state onto the opposite side of the latch so thatboth sides of the latch have the next state to be latched when NU goeslow. In this example, the NU port does not in any way impact the otherdata port controlled by the clock. On a dual port FF, having both portsactive at the same time is an illegal control condition, and theresulting port conflict means the resulting next state will beindeterminate. To avoid a port conflict, the system holds the clock inthe inactive state if the slave state is updated while in functionalmode. In retention mode, the RET signal along with supporting circuitsinside the FF are used to prevent electrical conflicts independent ofthe state of CLK while in retention mode (see the inverter controlled byRETB in the master stage).

As illustrated these additional elements are disposed in the slave stage222 of the associated FF. The additional transistors, however, are noton the critical path of the FF and have only 1.8% and 6.9% impact onnormal FF performance and power (simulation data) in this particularimplementation. When data from the NVL array is valid on the ND(NVL-Data) port, the NU (NVL-Update) control input is pulsed high for acycle to write to the FF. The thirty-one bit data output of an NVL arrayfans out to ND ports of eight thirty-one bit FF groups.

To save flip-flop state, a multiplexer is configured to pass states froma plurality of the individual ones of the plurality of volatile storageelements 220 for essentially simultaneous storage in an individual oneof the plurality of NVL arrays 110. For instance, the multiplexer may beconfigured to connect to N groups of M volatile storage elements of theplurality of volatile storage elements per group and to an N by M sizeNVL array of the plurality of NVL arrays. In this configuration, themultiplexer connects one of the N groups to the N by M size NVL array tostore data from the M volatile storage elements into a row of the N by Msize NVL array at one time. In the illustrated example, Q outputs of 248FFs are connected to the 31b parallel data input of NVL array 110through a 31b wide 8-1 mux 212. To minimize FF loading, the mux may bebroken down into smaller muxes based on the layout of the FF cloud andplaced close to the FFs they serve. Again, the NVL controllersynchronizes writing to the NVL array, and the select signals MUX_SEL<2:0> of 8-1 mux 212.

When the FFs are operating in a retention mode, a clock CLK of thecomputing device is a “don't care” such that it is irrelevant for thevolatile storage elements with respect to updating the slave stage statewhenever the NU signal is active, whereby the non-volatile logiccontroller is configured to control and effect storage of data fromindividual ones of the volatile storage elements into individual ones ofthe non-volatile storage elements. In other words, the clock CLK controlis not needed during NVL data recovery during retention mode, but theclock CLK should be controlled at the system level once the system stateis restored, right before the transition between retention mode andfunctional mode. In another approach, the NVL state can be recovered tothe volatile storage elements when the system is in a functional mode.In this situation where the VDDL power is active, the clock CLK is heldin the inactive state for the volatile storage elements during the datarestoration from the NVL array, whereby the non-volatile logiccontroller is configured to control and effect transfer of data fromindividual ones of the non-volatile storage elements into individualones of the volatile storage elements. For example, a system clock CLKis typically held low for positive edge FF based logic and held high fornegative edge FF based logic.

Generally speaking, to move from regular operation into system backupmode, the first step is to stop the system clock(s) in an inactive stateto freeze the machine state to not change while the backup is inprogress. The clocks are held in the inactive state until backup iscomplete. After backup is complete, all power domains are powered downand the state of the clock becomes a don't care in sleep mode bydefinition.

When restoring the state from NVL arrays, the FF are placed in aretention state (see Table 2 below) in which the clock continues to be adon't care as long as the RET signal is active (clock can be a don'tcare by virtue of special transistors added to each retention FF and iscontrolled by the RET signal). While restoring NVL state, the flopsremain in retention mode so clock remains a don't care. Once the NVLstate is recovered, the state of the machine logic that controls thestate of the system clocks will also be restored to the state they werein at the time of the state backup, which also means that for thisexample all the controls (including the volatile storage elements orFF's) that placed the system clock into inactive states have now beenrestored such that the system clocks will remain in the inactive stateupon completion of NVL data recovery. Now the RET signal can bedeactivated, and the system will sit quiescent with clocks deactivateduntil the NVL controller signals to the power management controller thatthe restoration is complete, in response to which the power managementcontroller will enable the clocks again.

To restore flip-flop state during restoration, NVL controller 106 readsan NVL row in NVL array 110 and then pulses the NU signal for theappropriate flip-flop group. During system restore, retention signal RETis held high and the slave latch is written from ND with power domainVDDL unpowered; at this point the state of the system clock CLK is adon't care. FF's are placed in the retention state with VDDL=0V andVDDR=VDD in order to suppress excess power consumption related tospurious data switching that occurs as each group of 31 FF's is updatedduring NVL array read operations. Suitably modified non-retention flopscan be used in NVL based SOC's at the expense of higher powerconsumption during NVL data recovery operations.

System clock CLK should start from low once VDDL comes up and thereafternormal synchronous operation continues with updated information in theFFs. Data transfer between the NVL arrays and their respective FFs canbe done in serial or parallel or any combination thereof to tradeoffpeak current and backup/restore time. Because a direct access isprovided to FFs controlled by at least one non-volatile logic controllerthat is separate from a central processing unit for the computing deviceapparatus, intervention from a microcontroller processing unit (CPU) isnot required for NVL operations; therefore the implementation is SoC/CPUarchitecture agnostic. Table 2 summarizes operation of the NVL flipflops.

TABLE 2 NVL Flip Flop truth table Clock Retention NVL update mode (CLK)(RET) (NU) Value saved Regular Pulsed 0 0 From D input operationretention X 1 0 Q value NVL system 0 0 0 From Q output backup NVL systemX 1 pulsed NVL cell bit data restore (ND)

Because the at least one non-volatile logic controller is configured tovariably control data transfer to or reading from the plurality ofnon-volatile arrays in parallel, sequentially, or in any combinationthereof based on input signals, system designers have additional optionswith respect to tailoring system operation specifications to particularneeds. For instance, because no computation can occur on an MCU SOCduring the time the system enters a low power system state or to wakeupfrom a low power state, minimizing the wakeup or go to sleep time isadvantageous. On the other hand, non-volatile state retention is powerintensive because significant energy is needed to save and restore stateto or from non-volatile elements such as ferro-electric capacitors. Thepower required to save and restore system state can exceed the capacityof the power delivery system and cause problems such as electromigrationinduced power grid degradation, battery life reduction due to excessivepeak current draw, or generation of high levels of noise on the powersupply system that can degrade signal integrity on die. Thus, allowing asystem designer to be able to balance between these two concerns isdesirable.

In one such approach, the at least one non-volatile logic controller 106is configured to receive the input signals through a user interface 125,such as those known to those of skill in the art. In another approach,the at least one non-volatile logic controller is configured to receivethe input signals from a separate computing element 130 that may beexecuting an application. In one such approach, the separate computingelement is configured to execute the application to determine a readingsequence for the plurality of non-volatile arrays based at least in parton a determination of power and computing resource requirements for thecomputing device apparatus 130. So configured, a system user canmanipulate the system state store and retrieve procedure to fit a givendesign.

FIG. 3 is a plot illustrating polarization hysteresis exhibited by aferroelectric capacitor. The general operation of ferroelectric bitcells is known. When most materials are polarized, the polarizationinduced, P, is almost exactly proportional to the applied externalelectric field E; so the polarization is a linear function, referred toas dielectric polarization. In addition to being nonlinear,ferroelectric materials demonstrate a spontaneous nonzero polarizationas illustrated in FIG. 3 when the applied field E is zero. Thedistinguishing feature of ferroelectrics is that the spontaneouspolarization can be reversed by an applied electric field; thepolarization is dependent not only on the current electric field butalso on its history, yielding a hysteresis loop. The term“ferroelectric” is used to indicate the analogy to ferromagneticmaterials, which have spontaneous magnetization and also exhibithysteresis loops.

The dielectric constant of a ferroelectric capacitor is typically muchhigher than that of a linear dielectric because of the effects ofsemi-permanent electric dipoles formed in the crystal structure of theferroelectric material. When an external electric field is appliedacross a ferroelectric dielectric, the dipoles tend to align themselveswith the field direction, produced by small shifts in the positions ofatoms that result in shifts in the distributions of electronic charge inthe crystal structure. After the charge is removed, the dipoles retaintheir polarization state. Binary “0”s and “1”s are stored as one of twopossible electric polarizations in each data storage cell. For example,in the figure a “1” may be encoded using the negative remnantpolarization 302, and a “0” may be encoded using the positive remnantpolarization 304, or vice versa.

Ferroelectric random access memories have been implemented in severalconfigurations. A one transistor, one capacitor (1T-1C) storage celldesign in an FeRAM array is similar in construction to the storage cellin widely used DRAM in that both cell types include one capacitor andone access transistor. In a DRAM cell capacitor, a linear dielectric isused, whereas in an FeRAM cell capacitor the dielectric structureincludes ferroelectric material, typically lead zirconate titanate(PZT). Due to the overhead of accessing a DRAM type array, a 1T-1C cellis less desirable for use in small arrays such as NVL array 110.

A four capacitor, six transistor (4C-6T) cell is a common type of cellthat is easier to use in small arrays. An improved four capacitor cellwill now be described.

FIG. 4 is a schematic illustrating one embodiment of a ferroelectricnonvolatile bitcell 400 that includes four capacitors and twelvetransistors (4C-12T). The four FeCaps are arranged as two pairs in adifferential arrangement. FeCaps C1 and C2 are connected in series toform node Q 404, while FeCaps C1′ and C2′ are connected in series toform node QB 405, where a data bit is written into node Q and stored inFeCaps C1 and C2 via bit line BL and an inverse of the data bit iswritten into node QB and stored in FeCaps C1′ and C2′ via inversebitline BLB. Sense amp 410 is coupled to node Q and to node QB and isconfigured to sense a difference in voltage appearing on nodes Q, QBwhen the bitcell is read. The four transistors in sense amp 410 areconfigured as two cross coupled inverters to form a latch. Pass gate 402is configured to couple node Q to bitline B and pass gate 403 isconfigured to couple node QB to bit line BLB. Each pass gate 402, 403 isimplemented using a PMOS device and an NMOS device connected inparallel. This arrangement reduces voltage drop across the pass gateduring a write operation so that nodes Q, QB are presented with a highervoltage during writes and thereby a higher polarization is imparted tothe FeCaps. Plate line 1 (PL1) is coupled to FeCaps C1 and C1′ and plateline 2 (PL2) is coupled to FeCaps C2 and C2′. The plate lines are use toprovide biasing to the FeCaps during reading and writing operations.Alternatively, in another embodiment the cmos pass gates can be replacedwith NMOS pass gates that use a pass gate enable that is has a voltagehigher than VDDL. The magnitude of the higher voltage must be largerthan the usual NMOS Vt in order to pass a undegraded signal from thebitcell Q/QB nodes to/from the bitlines BL/BLB (I.E. Vpass_gate_controlmust be >VDDL+Vt).

Typically, there will be an array of bit cells 400. There may then bemultiple columns of similar bitcells to form an n row by m column array.For example, in SoC 100, the NVL arrays are 8×32; however, as discussedearlier, different configurations may be implemented.

FIGS. 5 and 6 are timing diagram illustrating read and write waveformsfor reading a data value of logical 0 and writing a data value oflogical 0, respectively. Reading and writing to the NVL array is amulti-cycle procedure that may be controlled by the NVL controller andsynchronized by the NVL clock. In another embodiment, the waveforms maybe sequenced by fixed or programmable delays starting from a triggersignal, for example. During regular operation, a typical 4C-6T bitcellis susceptible to time dependent dielectric breakdown (TDDB) due to aconstant DC bias across FeCaps on the side storing a “1”. In adifferential bitcell, since an inverted version of the data value isalso stored, one side or the other will always be storing a “1”.

To avoid TDDB, plate line PL1, plate line PL2, node Q and node QB areheld at a quiescent low value when the cell is not being accessed, asindicated during time periods s0 in FIGS. 5, 6. Power disconnecttransistors MP 411 and MN 412 allow sense amp 410 to be disconnectedfrom power during time periods s0 in response to sense amp enablesignals SAEN and SAENB. Clamp transistor MC 406 is coupled to node Q andclamp transistor MC′ 407 is coupled to node QB. Clamp transistors 406,407 are configured to clamp the Q and QB nodes to a voltage that isapproximately equal to the low logic voltage on the plate lines inresponse to clear signal CLR during non-access time periods s0, which inthis embodiment equal 0 volts, (the ground potential). In this manner,during times when the bit cell is not being accessed for reading orwriting, no voltage is applied across the FeCaps and therefore TDDB isessentially eliminated. The clamp transistors also serve to prevent anystray charge buildup on nodes Q and QB due to parasitic leakagecurrents. Build up of stray charge may cause the voltage on Q or QB torise above 0 v, leading to a voltage differential across the FeCapsbetween Q or QB and PL1 and PL2. This can lead to unintendeddepolarization of the FeCap remnant polarization and could potentiallycorrupt the logic values stored in the FeCaps.

In this embodiment, Vdd is 1.5 volts and the ground reference plane hasa value of 0 volts. A logic high has a value of approximately 1.5 volts,while a logic low has a value of approximately 0 volts. Otherembodiments that use logic levels that are different from ground forlogic 0 (low) and Vdd for logic 1 (high) would clamp nodes Q, QB to avoltage corresponding to the quiescent plate line voltage so that thereis effectively no voltage across the FeCaps when the bitcell is notbeing accessed.

In another embodiment, two clamp transistors may be used. Each of thesetwo transistors is used to clamp the voltage across each FeCap to be nogreater than one transistor Vt (threshold voltage). Each transistor isused to short out the FeCaps. In this case, for the first transistor,one terminal connects to Q and the other one connects to PL1, while fortransistor two, one terminal connects to Q and the other connects toPL2. The transistor can be either NMOS or PMOS, but NMOS is more likelyto be used.

Typically, a bit cell in which the two transistor solution is used doesnot consume significantly more area than the one transistor solution.The single transistor solution assumes that PL1 and PL2 will remain atthe same ground potential as the local VSS connection to the singleclamp transistor, which is normally a good assumption. However, noise orother problems may occur (especially during power up) that might causePL1 or PL2 to glitch or have a DC offset between the PL1/PL2 driveroutput and VSS for brief periods; therefore, the two transistor designmay provide a more robust solution.

To read bitcell 400, plate line PL1 is switched from low to high whilekeeping plate line PL2 low, as indicated in time period s2. This inducesvoltages on nodes Q, QB whose values depend on the capacitor ratiobetween C1-C2 and C1′-C2′ respectively. The induced voltage in turndepends on the remnant polarization of each FeCap that was formed duringthe last data write operation to the FeCap's in the bit cell. Theremnant polarization in effect “changes” the effective capacitance valueof each FeCap which is how FeCaps provide nonvolatile storage. Forexample, when a logic 0 was written to bitcell 400, the remnantpolarization of C2 causes it to have a lower effective capacitancevalue, while the remnant polarization of C1 causes it to have a highereffective capacitance value. Thus, when a voltage is applied acrossC1-C2 by switching plate line PL1 high while holding plate line PL2 low,the resultant voltage on node Q conforms to equation (1). A similarequation holds for node QB, but the order of the remnant polarization ofC1′ and C2′ is reversed, so that the resultant voltages on nodes Q andQB provide a differential representation of the data value stored in bitcell 400, as illustrated at 502, 503 in FIG. 5.

$\begin{matrix}{{V(Q)} = {{V\left( {{PL}\; 1} \right)}\left( \frac{C\; 2}{{C\; 1} + {C\; 2}} \right)}} & (1)\end{matrix}$

The local sense amp 410 is then enabled during time period s3. Aftersensing the differential values 502, 503, sense amp 410 produces a fullrail signal 504, 505. The resulting full rail signal is transferred tothe bit lines BL, BLB during time period s4 by asserting the transfergate enable signals PASS, PASSB to enable transfer gates 402, 403 andthereby transfer the full rail signals to an output latch responsive tolatch enable signal LAT_EN that is located in the periphery of NVL array110, for example

FIG. 6 is a timing diagram illustrating writing a logic 0 to bit cell400. The write operation begins by raising both plate lines to Vddduring time period s1. This is called the primary storage method. Thesignal transitions on PL1 and PL2 are capacitively coupled onto nodes Qand QB, effectively pulling both storage nodes almost all the way to VDD(1.5 v). Data is provided on the bit lines BL, BLB and the transfergates 402, 403 are enabled by the pass signal PASS during time periodss2-s4 to transfer the data bit and its inverse value from the bit linesto nodes Q, QB. Sense amp 410 is enabled by sense amp enable signalsSAEN, SAENB during time period s3, s4 to provide additional drive afterthe write data drivers have forced adequate differential on Q/QB duringtime period s2. However, to avoid a short from the sense amp to the 1.2v driver supply, the write data drivers are turned off at the end oftime period s2 before the sense amp is turned on during time periods s3,s4. In an alternative embodiment called the secondary store method,write operations hold PL2 at 0 v or ground throughout the data writeoperation. This can save power during data write operations, but reducesthe resulting read signal margin by 50% as C2 and C2′ no longer holddata via remnant polarization and only provide a linear capacitive loadto the C1 and C2 FeCaps.

Key states such as PL1 high to SAEN high during s2, SAEN high pulseduring s3 during read and FeCap DC bias states s3-4 during write canselectively be made multi-cycle to provide higher robustness withoutslowing down the NVL clock.

For FeCap based circuits, reading data from the FeCap's may partiallydepolarize the capacitors. For this reason, reading data from FeCaps isconsidered destructive in nature; i.e. reading the data may destroy thecontents of the FeCap's or reduce the integrity of the data at aminimum. For this reason, if the data contained in the FeCap's isexpected to remain valid after a read operation has occurred, the datamust be written back into the FeCaps.

In certain applications, specific NVL arrays may be designated to storespecific information that will not change over a period of time. Forexample, certain system states can be saved as a default return statewhere returning to that state is preferable to full reboot of thedevice. The reboot and configuration process for a state of the artultra low power SoC can take 1000-10000 clock cycles or more to reachthe point where control is handed over to the main application codethread. This boot time becomes critical for energy harvestingapplications in which power is intermittent, unreliable, and limited inquantity. The time and energy cost of rebooting can consume most or allof the energy available for computation, preventing programmable devicessuch as MCU's from being used in energy harvesting applications. Anexample application would be energy harvesting light switches. Theenergy harvested from the press of the button on the light switchrepresents the entire energy available to complete the followingtasks: 1) determine the desired function (on/off or dimming level), 2)format the request into a command packet, 3) wake up a radio and squirtthe packet over an RF link to the lighting system. Known custom ASICchips with hard coded state machines are often used for this applicationdue to the tight energy constraints, which makes the system inflexibleand expensive to change because new ASIC chips have to be designed andfabricated whenever any change is desired. A programmable MCU SOC wouldbe a much better fit, except for the power cost of the boot processconsumes most of the available energy, leaving no budget for executingthe required application code.

To address this concern, in one approach, at least one of the pluralityof non-volatile logic element arrays is configured to store a boot staterepresenting a state of the computing device apparatus after a givenamount of a boot process is completed. The at least one non-volatilelogic controller in this approach is configured to control restorationof data representing the boot state from the at least one of theplurality of non-volatile logic element arrays to corresponding ones ofthe plurality of volatile storage elements in response to detecting aprevious system reset or power loss event for the computing deviceapparatus. To conserve power over a typical read/write operation for theNVL arrays, the at least one non-volatile logic controller can beconfigured to execute a round-trip data restoration operation thatautomatically writes back data to an individual non-volatile logicelement after reading data from the individual non-volatile logicelement without completing separate read and write operations.

An example execution of a round-trip data restoration is illustrated inFIG. 7, which illustrates a writeback operation on bitcell 400, wherethe bitcell is read, and then written to the same value. As illustrated,initiating reading of data from the individual non-volatile logicelement is started at a first time S1 by switching a first plate linePL1 high to induce a voltage on a node of a corresponding ferroelectriccapacitor bit cell based on a capacitance ratio for ferroelectriccapacitors of the corresponding ferroelectric capacitor bit cell. Ifclamp switches are used to ground the nodes of the ferroelectriccapacitors, a clear signal CLR is switched from high to low at the firsttime S1 to unclamp those aspects of the individual non-volatile logicelement from electrical ground. At a second time S2, a sense amplifierenable signal SAEN is switched high to enable a sense amplifier todetect the voltage induced on the node and to provide an output signalcorresponding to data stored in the individual non-volatile logicelement. At a third time S3, a pass line PASS is switched high to opentransfer gates to provide an output signal corresponding to data storedin the individual non-volatile logic element. At a fourth time S4, asecond plate line PL2 is switched high to induce a polarizing signalacross the ferroelectric capacitors to write data back to thecorresponding ferroelectric capacitor bit cell corresponding to the datastored in the individual non-volatile logic element. To the individualnon-volatile logic element to a non-volatile storage state having thesame data stored therein, at a fifth time S5 the first plate line PL1and the second plate line PL2 are switched low, the pass line PASS isswitched low at the sixth time S6, and the sense amplifier enable signalSAEN is switched law at the seventh time S7. If clamp switches are usedto ground the nodes of the ferroelectric capacitors, at the seventh timea clear signal CLR is switched from low to high to clamp the aspects ofthe individual non-volatile logic element to the electrical ground tohelp maintain data integrity as discussed herein. This process includesa lower total number of transitions than what is needed for distinct andseparate read and write operations (read, then write). This lowers theoverall energy consumption.

Bitcell 400 is designed to maximize read differential across Q/QB inorder to provide a highly reliable first generation of NVL products. TwoFeCaps are used on each side rather than using one FeCap and constant BLcapacitance as a load because this doubles the differential voltage thatis available to the sense amp. A sense amp is placed inside the bitcellto prevent loss of differential due to charge sharing between node Q andthe BL capacitance and to avoid voltage drop across the transfer gate.The sensed voltages are around VDD/2, and a HVT transfer gate takes along time to pass them to the BL. Bitcell 400 helps achieve twice thesignal margin of a regular FRAM bitcell known in the art, while notallowing any DC stress across the FeCaps.

The timing of signals shown in FIGS. 5 and 6 are for illustrativepurposes. Various embodiments may signal sequences that vary dependingon the clock rate, process parameters, device sizes, etc. For example,in another embodiment, the timing of the control signals may operate asfollows. During time period S1: PASS goes from 0 to 1 and PL1/PL2 gofrom 0 to 1. During time period S2: SAEN goes from 0 to 1, during whichtime the sense amp may perform level shifting as will be describedlater, or provides additional drive strength for a non-level shifteddesign. During time period S3: PL1/PL2 go from 1 to 0 and the remainderof the waveforms remain the same, but are moved up one clock cycle. Thissequence is one clock cycle shorter than that illustrated in FIG. 6.

In another alternative, the timing of the control signals may operate asfollows. During time period S1: PASS goes from 0 to 1 (BL/BLB, Q/QB are0 v and VDDL respectively). During time period S2: SAEN goes from 0 to 1(BL/BLB, Q/QB are 0 v and VDDN respectively). During time period S3:PL1/PL2 go from 0 to 1 (BL/Q is coupled above ground by PL1/PL2 and isdriven back low by the SA and BL drivers). During time period S4:PL1/PL2 go from 1 to 0 and the remainder of the waveforms remain thesame.

FIGS. 8-9 are a schematic and timing diagram illustrating anotherembodiment of a ferroelectric nonvolatile bit cell 800, a 2C-3Tself-referencing based NVL bitcell. The previously described 4-FeCapbased bitcell 400 uses two FeCaps on each side of a sense amp to get adifferential read with double the margin as compared to a standard 1C-1TFRAM bitcell. However, a 4-FeCap based bitcell has a larger area and mayhave a higher variation because it uses more FeCaps.

Bitcell 800 helps achieve a differential 4-FeCap like margin in lowerarea by using itself as a reference, referred to herein asself-referencing. By using fewer FeCaps, it also has lower variationthan a 4 FeCap bitcell. Typically, a single sided cell needs to use areference voltage that is in the middle of the operating range of thebitcell. This in turn reduces the read margin by half as compared to atwo sided cell. However, as circuit fabrication process moves, thereference value may become skewed, further reducing the read margin. Aself-reference scheme allows comparison of a single sided cell againstitself, thereby providing a higher margin. Tests of the self-referencingcell described herein have provided at least double the margin over afixed reference cell.

Bitcell 800 has two FeCaps C1, C2 that are connected in series to formnode Q 804. Plate line 1 (PL1) is coupled to FeCap C1 and plate line 2(PL2) is coupled to FeCap C2. The plate lines are use to provide biasingto the FeCaps during reading and writing operations. Pass gate 802 isconfigured to couple node Q to bitline B. Pass gate 802 is implementedusing a PMOS device and an NMOS device connected in parallel. Thisarrangement reduces voltage drop across the pass gate during a writeoperation so that nodes Q, QB are presented with a higher voltage duringwrites and thereby a higher polarization is imparted to the FeCaps.Alternatively, an NMOS pass gate may be used with a boosted word linevoltage. In this case, the PASS signal would be boosted by one NFET Vt(threshold voltage). However, this may lead to reliability problems andexcess power consumption. Using a CMOS pass gate adds additional area tothe bit cell but improves speed and power consumption. Clamp transistorMC 806 is coupled to node Q. Clamp transistor 806 is configured to clampthe Q node to a voltage that is approximately equal to the low logicvoltage on the plate lines in response to clear signal CLR duringnon-access time periods s0, which in this embodiment 0 volts, ground. Inthis manner, during times when the bit cell is not being accessed forreading or writing, no voltage is applied across the FeCaps andtherefore TDDB and unintended partial depolarization is essentiallyeliminated.

The initial state of node Q, plate lines PL1 and PL2 are all 0, as shownin FIG. 9 at time period s0, so there is no DC bias across the FeCapswhen the bitcell is not being accessed. To begin a read operation, PL1is toggled high while PL2 is kept low, as shown during time period s1. Asignal 902 develops on node Q from a capacitance ratio based on theretained polarization of the FeCaps from a last data value previouslywritten into the cell, as described above with regard to equation 1.This voltage is stored on a read capacitor 820 external to the bitcellby passing the voltage though transfer gate 802 onto bit line BL andthen through transfer gate 822 in response to a second enable signalEN1. Note: BL and the read capacitors are precharged to VDD/2 before thepass gates 802, 822, and 823 are enabled in order to minimize signalloss via charge sharing when the recovered signals on Q are transferredvia BL to the read storage capacitors 820 and 821. Then, PL1 is toggledback low and node Q is discharged using clamp transistor 806 during timeperiod s2. Next, PL2 is toggled high keeping PL1 low during time periods3. A new voltage 904 develops on node Q, but this time with theopposite capacitor ratio. This voltage is then stored on anotherexternal read capacitor 821 via transfer gate 823. Thus, the same twoFeCaps are used to read a high as well as low signal. Sense amplifier810 can then determine the state of the bitcell by using the voltagesstored on the external read capacitors 820, 821.

Typically, there will be an array of bit cells 800. One column of bitcells 800-800 n is illustrated in FIG. 8 coupled via bit line 801 toread transfer gates 822, 823. There may then be multiple columns ofsimilar bitcells to form an n row by m column array. For example, in SoC100, the NVL arrays are 8×32; however, as discussed earlier, differentconfigurations may be implemented. The read capacitors and sense ampsmay be located in the periphery of the memory array, for example.

FIG. 10 is a block diagram illustrating NVL array 110 in more detail.Embedding non-volatile elements outside the controlled environment of alarge array presents reliability and fabrication challenges. Asdiscussed earlier with reference to FIG. 1, adding testability featuresto individual NVL FFs may be prohibitive in terms of area overhead. Toamortize the test feature costs and improve manufacturability, SoC 100is implemented using 256b mini-NVL arrays 110, of FeCap based bitcellsdispersed throughout the logic cloud to save state of the various flipflops 120 when power is removed. Each cloud 102-104 of FFs 120 includesan associated NVL array 110. A central NVL controller 106 controls allthe arrays and their communication with FFs 120.

While an NVL array may be implemented in any number of n rows of mcolumn configurations, in this example, NVL array 110 is implementedwith an array 1040 of eight rows and thirty-two bit columns of bitcells.Each individual bit cell, such as bitcell 1041, is coupled to a set ofcontrol lines provided by row drivers 1042. The control signalsdescribed earlier, including plate lines (PL1, PL2), sense amp enable(SEAN), transfer gate enable (PASS), and clear (CLR) are all driven bythe row drivers. There is a set of row drivers for each row of bitcells.

Each individual bit cell, such as bitcell 1041 is also coupled via thebitlines to a set of input/output (IO) drivers 1044. In thisimplementation, there are thirty-two sets of IO drivers, such as IOdriver set 1045. Each driver set produces an output signal 1047 thatprovides a data value when a row of bit lines is read. Each bitline runsthe length of a column of bitcells and couples to an IO driver for thatcolumn. Each bitcell may be implemented as 2C-3T bitcell 800, forexample. In this case, a single bitline will be used for each column,and the sense amps and read capacitors will be located in IO driverblock 1044. In another implementation of NVL array 110, each bitcell maybe implemented as 4C-12T bit cell 400. In this case, the bitlines willbe a differential pair with two IO drivers for each column. A comparatorreceives the differential pair of bitlines and produces a final singlebit line that is provided to the output latch. Other implementations ofNVL array 110 may use other known or later developed bitcells inconjunction with the row drivers and IO drivers that will be describedin more detail below.

Timing logic 1046 generates timing signals that are used to control theread drivers to generate the sequence of control signals for each readand write operation. Timing logic 1046 may be implemented using bothsynchronous or asynchronous state machines, or other known or laterdeveloped logic technique. One potential alternative embodiment utilizesa delay chain with multiple outputs that “tap” the delay chain atdesired intervals to generate control signals. Multiplexors can be usedto provide multiple timing options for each control signal. Anotherpotential embodiment uses a programmable delay generator that producesedges at the desired intervals using dedicated outputs that areconnected to the appropriate control signals.

FIG. 11 is a more detailed schematic of a set of input/output circuits1150 used in the NVL array of FIG. 10. Referring back to FIG. 10, eachIO set 1045 of the thirty-two drivers in IO block 1044 is similar to IOcircuits 1150. I/O block 1044 provides several features to aidtestability of NVL bits.

Referring now to FIG. 11, a first latch (L1) 1151 serves as an outputlatch during a read and also combines with a second latch (L2) 1152 toform a scan flip flop. The scan output (SO) signal is routed tomultiplexor 1153 in the write driver block 1158 to allow writing scanneddata into the array during debug. Scan output (SO) is also coupled tothe scan input (SI) of the next set of IO drivers to form a thirty-twobit scan chain that can be used to read or write a complete row of bitsfrom NVL array 110. Within SoC 100, the scan latch of each NVL array isconnected in a serial manner to form a scan chain to allow all of theNVL arrays to be accessed using the scan chain. Alternatively, the scanchain within each NVL array may be operated in a parallel fashion (Narrays will generate N chains) to reduce the number of internal scanflop bits on each chain in order to speed up scan testing. The number ofchains and the number of NVL arrays per chain may be varied as needed.Typically, all of the storage latches and flipflops within SoC 100include scan chains to allow complete testing of SoC 100. Scan testingis well known and does not need to be described in more detail herein.In this embodiment, the NVL chains are segregated from the logic chainson a chip so that the chains can be exercised independently and NVLarrays can be tested without any dependencies on logic chainorganization, implementation, or control. The maximum total length ofNVL scan chains will always be less than the total length of logicchains since the NVL chain length is reduced by a divisor equal to thenumber of rows in the NVL arrays. In the current embodiment, there are 8entries per NVL array, so the total length of NVL scan chains is ⅛^(th)the total length of the logic scan chains. This reduces the timerequired to access and test NVL arrays and thus reduces test cost. Also,it eliminates the need to determine the mapping between logic flops,their position on logic scan chains and their corresponding NVL arraybit location (identifying the array, row, and column location), greatlysimplifying NVL test, debug, and failure analysis.

While scan testing is useful, it does not provide a good mechanism forproduction testing of SoC 100 since it may take a significant amount oftime to scan in hundreds or thousands of bits for testing the variousNVL arrays within SoC 100. This is because there is no direct access tobits within the NVL array. Each NVL bitcell is coupled to an associatedflip-flop and is only written to by saving the state of the flip flop.Thus, in order to load a pattern test into an NVL array from theassociated flipflops, the corresponding flipflops must be set up using ascan chain. Determining which bits on a scan chain have to be set orcleared in order to control the contents of a particular row in an NVLarray is a complex task as the connections are made based on thephysical location of arbitrary groups of flops on a silicon die and notbased on any regular algorithm. As such, the mapping of flops to NVLlocations need not be controlled and is typically somewhat random.

An improved testing technique is provided within IO drivers 1150. NVLcontroller 106, referring back to FIG. 1, has state machine(s) toperform fast pass/fail tests for all NVL arrays on the chip to screenout bad dies. In one such approach, at least one non-volatile logiccontroller is configured to control a built-in-self-test mode where allzeros or all ones are written to at least a portion of an NVL array ofthe plurality of NVL arrays and then it is determined whether data readfrom the at least the portion of the NVL array is all ones or all zeros.This is done by first writing all 0's or 1's to a row using all 0/1write driver 1180, applying an offset disturb voltage (V_Off), thenreading the same row using parallel read test logic 1170. Signalcorr_(—)1 from AND gate G1 goes high if the data output signal (OUT)from data latch 1151 is high, and signal corr_(—)1 from an adjacentcolumn's IO driver's parallel read test logic AND gate G1 is high. Inthis manner, the G1 AND gates of the thirty-two sets of I/O blocks 1150in NVL array 110 implement a large 32 input AND gate that tell the NVLcontroller if all outputs are high for the selected row of NVL array110. OR gate G0 does the same for reading 0's. In this manner, the NVLcontroller may instruct all of the NVL arrays within SoC 100 tosimultaneously perform an all ones write to a selected row, and theninstruct all of the NVL arrays to simultaneously read the selected rowand provide a pass fail indication using only a few control signalswithout transferring any explicit test data from the NVL controller tothe NVL arrays. In typical memory array BIST (Built In Self Test)implementations, the BIST controller must have access to all memoryoutput values so that each output bit can be compared with the expectedvalue. Given there are many thousands of logic flops on typical siliconSOC chips, the total number of NVL array outputs can also measure in thethousands. It would be impractical to test these arrays using normalBIST logic circuits due to the large number of data connections and datacomparators required. The NVL test method can then be repeated eighttimes, for NVL arrays having eight rows (the number of repetitions willvary according to the array organization. In one example, a 10 entry NVLarray implementation would repeat the test method 10 times), so that allof the NVL arrays in SoC 100 can be tested for correct all onesoperation in only eight write cycles and eight read cycles. Similarly,all of the NVL arrays in SoC 100 can be tested for correct all zerosoperation in only eight write cycles and eight read cycles. The resultsof all of the NVL arrays may be condensed into a single signalindicating pass or fail by an additional AND gate and OR gate thatreceive the corr_(—)0 and corr_(—)1 signals from each of the NVL arraysand produces a single corr_(—)0 and corr_(—)1 signal, or the NVLcontroller may look at each individual corr_(—)0 and corr_(—)1 signal.

All 0/1 write driver 1180 includes PMOS devices M1, M3 and NMOS devicesM2, M4. Devices M1 and M2 are connected in series to form a node that iscoupled to the bitline BL, while devices M3 and M4 are connected inseries to form a node that is coupled to the inverse bitline BLB.Control signal “all_(—)1_A” and inverse “all_(—)1_B” are generated byNVL controller 106. When asserted during a write cycle, they activatedevice devices M1 and M4 to cause the bit lines BL and BLB to be pulledto represent a data value of logic 1. Similarly, control signal“all_(—)0_A” and inverse “all_(—)0_B” are generated by NVL controller106. When asserted during a write cycle, they activate devices M2 and M3to cause the bit lines BL and BLB to be pulled to represent a data valueof logic 0. In this manner, the thirty-two drivers are operable to writeall ones into a row of bit cells in response to a control signal and towrite all zeros into a row of bit cells in response to another controlsignal. One skilled in the art can easily design other circuittopologies to accomplish the same task. The current embodiment ispreferred as it only requires 4 transistors to accomplish the requireddata writes.

During a normal write operation, write driver block 1158 receives a databit value to be stored on the data in signal. Write drivers 1156, 1157couple complimentary data signals to bitlines BL, BLB and thereby to theselected bit cell. Write drivers 1156, 1157 are enabled by the writeenable signal STORE.

FIG. 12A is a timing diagram illustrating an offset voltage test duringa read cycle. To apply a disturb voltage to a bitcell, state s1 ismodified during a read. This figure illustrates a voltage disturb testfor reading a data value of “0” (node Q); a voltage disturb test for adata value of “1” is similar, but injects the disturb voltage onto theopposite side of the sense amp (node QB). Thus, the disturb voltage inthis embodiment is injected onto the low voltage side of the sense ampbased on the logic value being read. Transfer gates 1154, 1155 arecoupled to the bit line BL, BLB. A digital to analog converter, notshown (may be on-chip, or off-chip in an external tester, for example),is programmed by NVL controller 106, by an off-chip test controller, orvia a external production tester to produce a desired amount of offsetvoltage V_OFF. NVL controller 106 may assert the Vcon control signal forthe bitline side storing a “0” during the s1 time period to therebyenable Vcon transfer gate 1154, 1155, discharge the other bit-line usingM2/M4 during s1, and assert control signal PASS during s1 to turn ontransfer gates 402, 403. This initializes the voltage on node Q/QB ofthe “0” storing side to offset voltage V_Off, as shown at 1202. Thispre-charged voltage lowers the differential available to the SA durings3, as indicated at 1204, and thereby pushes the bitcell closer tofailure. For fast production testing, V_Off may be set to a requiredmargin value, and the pass/fail test using G0-1 may then be used toscreen out any failing die.

FIG. 12B illustrates a histogram generated during a sweep of offsetvoltage. Bit level failure margins can be studied by sweeping V_Off andscanning out the read data bits using a sequence of read cycles, asdescribed above. In this example, the worst case read margin is 550 mv,the mean value is 597 mv, and the standard deviation is 22 mv. In thismanner, the operating characteristics of all bit cells in each NVL arrayon an SoC may be easily determined.

As discussed above, embedding non-volatile elements outside thecontrolled environment of a large array presents reliability andfabrication challenges. The NVL bitcell should be designed for maximumread signal margin and in-situ testability as is needed for anyNV-memory technology. However, NVL implementation cannot rely on SRAMlike built in self test (BIST) because NVL arrays are distributed insidethe logic cloud. The NVL implementation described above includes NVLarrays controlled by a central NVL controller 106. While screening a diefor satisfactory behavior, NVL controller 106 runs a sequence of stepsthat are performed on-chip without any external tester interference. Thetester only needs to issue a start signal, and apply an analog voltagewhich corresponds to the desired signal margin. The controller firstwrites all 0s or is to all bits in the NVL array. It then starts readingan array one row at a time. The NVL array read operations do notnecessarily immediately follow NVL array write operations. Often, hightemperature bake cycles are inserted between data write operations anddata read operations in order to accelerate time and temperaturedependent failure mechanisms so that defects that would impact long termdata retention can be screened out during manufacturing related testing.As described above in more detail, the array contains logic that ANDsand ORs all outputs of the array. These two signals are sent to thecontroller. Upon reading each row, the controller looks at the twosignals from the array, and based on knowledge of what it previouslywrote, decides it the data read was correct or not in the presence ofthe disturb voltage. If the data is incorrect, it issues a fail signalto the tester, at which point the tester can eliminate the die. If therow passes, the controller moves onto the next row in the array. Allarrays can be tested in parallel at the normal NVL clock frequency. Thisenables high speed on-chip testing of the NVL arrays with the testeronly issuing a start signal and providing the desired read signal marginvoltage while the NVL controller reports pass at the end of the built intesting procedure or generates a fail signal whenever the first failingrow is detected. Fails are reported immediately so the tester can abortthe test procedure at the point of first failure rather than wasteadditional test time testing the remaining rows. This is important astest time and thus test cost for all non-volatile memories (NVM) oftendominates the overall test cost for an SOC with embedded NVM. If the NVLcontroller activates the “done” signal and the fail signal has not beenactivated at any time during the test procedure, the die undergoingtesting has passed the required tests.

For further failure analysis, the controller may also have a debug mode.In this mode, the tester can specify an array and row number, and theNVL controller can then read or write to just that row. The readcontents can be scanned out using the NVL scan chain. This methodprovides read or write access to any NVL bit on the die without CPUintervention or requiring the use of a long complicated SOC scan chainsin which the mapping of NVL array bits to individual flops is random.Further, this can be done in concert with applying an analog voltage forread signal margin determination, so exact margins for individual bitscan be measured.

These capabilities help make NVL practical because without testabilityfeatures it would be risky to use non-volatile logic elements in aproduct. Further, pass/fail testing on-die with minimal testerinteraction reduces test time and thereby cost.

NVL implementation using mini-arrays distributed in the logic cloudmeans that a sophisticated error detection method like ECC would requirea significant amount of additional memory columns and control logic tobe used on a per array basis, which could be prohibitive from an areastandpoint. However, in order to provide an enhanced level ofreliability, the NVL arrays of SoC 100 may include parity protection asa low cost error detection method, as will now be described in moredetail.

FIG. 13 is a schematic illustrating parity generation in NVL array 110that illustrates an example NVL array having thirty-two columns of bits(0:31), that exclusive-ors the input data value DATA_IN 1151 with theoutput of a similar XOR gate of the previous column's IO driver. Each IOdriver section, such as section 1350, of the NVL array may contain anXOR gate 1160, referring again to FIG. 11A. During a row write, theoutput of XOR gate 1160 that is in column 30 is the overall parity valueof the row of data that is being written in bit columns 0:30 and is usedto write parity values into the last column by feeding its output to thedata input of column 31 the NVL mini-array, shown as XOR_IN in FIG. 11B.

In a similar manner, during a read, XOR gate 1160 exclusive-ors the datavalue DATA_OUT from read latch 1151 via mux 1161 (see FIG. 11) with theoutput of a similar XOR gate of the previous column's IO driver. Theoutput of XOR gate 1160 that is in bit column 30 is the overall parityvalue for the row of data that was read from bit columns 0:30 and isused to compare to a parity value read from bit column 31 in parityerror detector 1370. If the overall parity value determined from theread data does not match the parity bit read from column 31, then aparity error is declared.

When a parity error is detected, it indicates that the stored FF statevalues are not trustworthy. Since the NVL array is typically being readwhen the SoC is restarting operation after being in a power off state,then detection of a parity error indicates that a full boot operationneeds to be performed in order to regenerate the correct FF statevalues.

However, if the FF state was not properly stored prior to turning offthe power or this is a brand new device, for example, then anindeterminate condition may exist. For example, if the NVL array isempty, then typically all of the bits may have a value of zero, or theymay all have a value of one. In the case of all zeros, the parity valuegenerated for all zeros would be zero, which would match the parity bitvalue of zero. Therefore, the parity test would incorrectly indicatethat the FF state was correct and that a boot operation is not required,when in fact it would be required. In order to prevent this occurrence,an inverted version of the parity bit may be written to column 31 by bitline driver 1365, for example. Referring again to FIG. 11A, note thatwhile bit line driver 1156 for columns 0-30 also inverts the input databits, mux 1153 inverts the data in bits when they are received, so theresult is that the data in columns 0-30 is stored un-inverted. Inanother embodiment, the data bits may be inverted and the parity errornot inverted, for example.

In the case of all ones, if there is an even number of columns, then thecalculated parity would equal zero, and an inverted value of one wouldbe stored in the parity column. Therefore, in an NVL array with an evennumber of data columns with all ones would not detect a parity error. Inorder to prevent this occurrence, NVL array 110 is constrained to havean odd number of data columns. For example, in this embodiment, thereare thirty-one data columns and one parity column, for a total ofthirty-two bitcell columns.

In some embodiments, when an NVL read operation occurs, control logicfor the NVL array causes the parity bit to be read, inverted, andwritten back. This allows the NVL array to detect when prior NVL arraywrites were incomplete or invalid/damaged. Remnant polarization is notcompletely wiped out by a single read cycle. Typically, it take 5-15read cycles to fully depolarize the FeCaps or to corrupt the data enoughto reliably trigger an NVL read parity. For example, if only four out ofeight NVL array rows were written during the last NVL store operationdue to loss of power, this would most likely result in an incompletecapture of the prior machine state. However, because of remnantpolarization, the four rows that were not written in the most recentstate storage sequence will likely still contain stale data from back intime, such as two NVL store events ago, rather than data from the mostrecent NVL data store event. The parity and stale data from the fourrows will likely be read as valid data rather than invalid data. This ishighly likely to cause the machine to lock up or crash when the machinestate is restored from the NVL arrays during the next wakeup/power upevent. Therefore, by writing back the parity bit inverted after everyentry is read, each row of stale data is essentially forciblyinvalidated.

Writing data back to NVL entries is power intensive, so it is preferableto not write data back to all bits, just the parity bit. The currentembodiment of the array disables the PL1, PL2, and sense amp enablesignals for all non-parity bits (i.e. Data bits) to minimize theparasitic power consumption of this feature.

In this manner, each time the SoC transitions from a no-power state to apower-on state, a valid determination can be made that the data beingread from the NVL arrays contains valid FF state information. If aparity error is detected, then a boot operation can be performed inplace of restoring FF state from the NVL arrays.

Referring back to FIG. 1, low power SoC 100 has multiple voltage andpower domains, such as VDDN_FV, VDDN_CV for the NVL arrays, VDDR for thesleep mode retention latches and well supplies, and VDDL for the bulk ofthe logic blocks that form the system microcontroller, variousperipheral devices, SRAM, ROM, etc., as described earlier with regard toTable 1 and Table 2. FRAM has internal power switches and is connectedto the always on supply VDDZ In addition, the VDDN_FV domain may bedesigned to operate at one voltage, such as 1.5 volts needed by theFeCap bit cells, while the VDDL and VDDN_CV domain may be designed tooperate at a lower voltage to conserve power, such as 0.9-1.5 volts, forexample. Such an implementation requires using power switches 108, levelconversion and isolation in appropriate areas. Aspects of isolation andlevel conversion needed with respect to NVL blocks 110 will now bedescribed in more detail. The circuits are designed such thatVDDL/VDDN_CV can be any valid voltage less than or equal to VDDN_FV andthe circuit will function correctly.

FIG. 14 is a block diagram illustrating power domains within NVL array110. Various block of logic and memory may be arranged as illustrated inTable 3.

TABLE 3 example full chip power domains Full Chip Voltage Voltage Domainlevel VDD 0.9-1.5 Always ON supply for VDDL, VDDR, VDDN_CV powerswitches, and always ON logic (if any) VDDZ 1.5 Always on 1.5 V supplyfor FRAM, and for VDDN_FV power switches. FRAM has internal powerswitches. VDDL 0.9-1.5 All logic, and master stage of all flops, SRAM,ROM, Write multiplexor, buffers on FF outputs, and mux outputs: Variablelogic voltage; e.g. 0.9 to 1.5 V (VDDL). This supply is derived from theoutput of VDDL power switches VDDN_CV 0.9-1.5 NVL array control andtiming logic, and IO circuits, NVL controller. Derived from VDDN_CVpower switches. VDDN_FV 1.5 NVL array Wordline driver circuits 1042 andNVL bitcell array 1040: Same voltage as FRAM. Derived from VDDN_FV powerswitches. VDDR 0.9-1.5 This is the data retention domain and includesthe slave stage of retention flops, buffers on NVL clock, flop retentionenable signal buffers, and NVL control outputs such as flop updatecontrol signal buffers, and buffers on NVL data outputs. Derived fromVDDR power switches.

Power domains VDDL, VDDN_CV, VDDN_FV, and VDDR described in Table 3 arecontrolled using a separate set of power switches, such as switches 108described earlier. However, isolation may be needed for some conditions.Data output buffers within IO buffer block 1044 are in the NVL logicpower domain VDDN_CV and therefore may remain off while domain VDDR (orVDDL depending on the specific implementation) is ON during normaloperation of the chip. ISO-Low isolation is implemented to tie all suchsignals to ground during such a situation. While VDDN_CV is off, logicconnected to data outputs in VDDR (or VDDL depending on the specificimplementation) domain in random logic area may generate short circuitcurrent between power and ground in internal circuits if any signalsfrom the VDDN_CV domain are floating (not driven when VDDN_CV domain ispowered down) if they are not isolated. The same is applicable forcorrect_(—)0/1 outputs and scan out output of the NVL arrays. Thegeneral idea here is that any outputs of the NVL array will be isolatedwhen the NVL array has no power given to it. In case there is always ONlogic present in the chip, all signals going from VDDL or VDDN_CV to VDDmust be isolated using input isolation at the VDD domain periphery.Additional built-in isolation exists in NVL flops at the ND input. Here,the input goes to a transmission gate, whose control signal NU is drivenby an always on signal. When the input is expected to be indeterminate,NU is made low, thereby disabling the ND input port. Similar built-inisolation exists on data inputs and scan-in of the NVL array. Thisisolation would be needed during NVL restore when VDDL is OFF.Additionally, signals NU and NVL data input multiplexor enable signals(mux sel) must be buffered only in the VDDR domain. The same applies forthe retention enable signal.

To enable the various power saving modes of operation, VDDL and VDDN*domain are shut off at various times, and isolation is makes thatpossible without burning short circuit current.

Level conversion from the lower voltage VDDL domain to the highervoltage VDDN domain is needed on control inputs of the NVL arrays thatgo to the NVL bitcells, such as: row enables, PL1, PL2, restore, recall,and clear, for example. This enables a reduction in system powerdissipation by allowing blocks of SOC logic and NVL logic gates that canoperate at a lower voltage to do so. For each row of bitcells in bitcellarray 1040, there is a set of word line drivers 1042 that drive thesignals for each row of bitcells, including plate lines PL1, PL2,transfer gate enable PASS, sense amp enable SAEN, clear enable CLR, andvoltage margin test enable VCON, for example. The bitcell array 1040 andthe wordline circuit block 1042 are supplied by VDDN. Level shifting oninput signals to 1042 are handled by dedicated level shifters (see FIG.15), while level shifting on inputs to the bitcell array 1040 arehandled by special sequencing of the circuits within the NVL bitcellswithout adding any additional dedicated circuits to the array datapathor bitcells.

FIG. 15 is a schematic of a level converter 1500 for use in NVL array110. FIG. 15 illustrates one wordline driver that may be part of the setof wordline drivers 1402. Level converter 1500 includes PMOS transistorsP1, P2 and NMOS transistor N1, N2 that are formed in region 1502 in the1.5 volt VDDN domain for wordline drivers 1042. However, the controllogic in timing and control module 1046 is located in region 1503 in the1.2 v VDDL domain (1.2 v is used to represent the variable VDDL coresupply that can range from 0.9 v to 1.5 v). 1.2 volt signal 1506 isrepresentative of any of the row control signals that are generated bycontrol module 1046, for use in accessing NVL bitcell array 1040.Inverter 1510 forms a complimentary pair of control signals 1511, 1512in region 1503 that are then routed to transistors N1 and N2 in levelconverter 1500. In operation, when 1.2 volt signal 1506 goes high, NMOSdevice N1 pulls the gate of PMOS device P2 low, which causes P2 to pullsignal 1504 up to 1.5 volts. Similarly, when 1.2 volt signal 1506 goeslow, complimentary signal 1512 causes NMOS device N2 to pull the gate ofPMOS device P1 low, which pulls up the gate of PMOS device P2 and allowssignal 1504 to go low, approximately zero volts. The NMOS devices mustbe stronger than the PMOS so the converter doesn't get stuck. In thismanner, level shifting may done across the voltage domains and power maybe saved by placing the control logic, including inverter 1510, in thelower voltage domain 1503. For each signal, the controller is coupled toeach of level converter 1500 by two complimentary control signals 1511,1512.

FIG. 16 is a timing diagram illustrating operation of level shiftingusing a sense amp within a ferroelectric bitcell. Input data that isprovided to NVL array 110 from multiplexor 212, referring again to FIG.2, also needs to be level shifted from the 1.2 v VDDL domain to 1.5volts needed for best operation of the FeCaps in the 1.5 volt VDDNdomain during write operations. This may be done using the sense amp ofbit cell 400, for example. Referring again to FIG. 4 and to FIG. 13,note that each bit line BL, such as BL 1352, which comes from the 1.2volt VDDL domain, is coupled to transfer gate 402 or 403 within bitcell400. Sense amp 410 operates in the 1.5 v VDDN power domain. Referringnow to FIG. 16, note that during time period s2, data is provided on thebit lines BL, BLB and the transfer gates 402, 403 are enabled by thepass signal PASS during time periods s2 to transfer the data bit and itsinverse value from the bit lines to differential nodes Q, QB. However,as shown at 1602, the voltage level transferred is only limited to lessthan the 1.5 volt level because the bit line drivers are located in the1.2 v VDDL domain.

Sense amp 410 is enabled by sense amp enable signals SAEN, SAENB duringtime period s3, s4 to provide additional drive, as illustrated at 1604,after the write data drivers, such as write driver 1156, 1157, haveforced adequate differential 1602 on Q/QB during time period s2. Sincethe sense amp is supplied by a higher voltage (VDDN), the sense amp willrespond to the differential established across the sense amp by thewrite data drivers and will clamp the logic 0 side of the sense amp toVSS (Q or QB) while the other side containing the logic 1 is pulled upto VDDN voltage level. In this manner, the existing NVL array hardwareis reused to provide a voltage level shifting function during NVL storeoperations.

However, to avoid a short from the sense amp to the 1.2 v driver supply,the write data drivers are isolated from the sense amp at the end oftime period s2 before the sense amp is turned on during time periods s3,s4. This may be done by turning off the bit line drivers by de-assertingthe STORE signal after time period s2 and/or also by disabling thetransfer gates by de-asserting PASS after time period s2.

Using the above described arrangements, various configurations arepossible to maximize power savings or usability at various points in aprocessing or computing devices operation cycle. In one such approach, acomputing device can be configured to operate continuously across aseries of power interruptions without loss of data or reboot. Withreference to the example illustrated in FIG. 17, a processing device1700 as described above includes a plurality of non-volatile logicelement arrays 1710, a plurality of volatile storage elements 1720, andat least one non-volatile logic controller 1730 configured to controlthe plurality of non-volatile logic element arrays 1710 to store amachine state represented by the plurality of volatile storage elements1720 and to read out a stored machine state from the plurality ofnon-volatile logic element arrays 1710 to the plurality of volatilestorage elements 1720. A voltage or current detector 1740 is configuredto sense a power quality from an input power supply 1750.

A power management controller 1760 is in communication with the voltageor current detector 1740 to receive information regarding the powerquality from the voltage or current detector 1710. The power managementcontroller 1760 is also configured to be in communication with the atleast one non-volatile logic controller 1710 to provide informationeffecting storing the machine state to and restoration of the machinestate from the plurality of non-volatile logic element arrays 1710.

A voltage regulator 1770 is connected to receive power from the inputpower supply 1750 and provide power to an output power supply rail 1755configured to provide power to the processing device 1700. The voltageregulator 1770 is further configured to be in communication with thepower management controller 1760 and to disconnect the output powersupply rail 1755 from the input power supply 1750, such as throughcontrol of a switch 1780, in response to a determination that the powerquality is below a threshold.

The power management controller 1760 and the voltage or current detector1740 work together with the at least one non-volatile logic controller1730 and voltage regulator 1770 to manage the data backup andrestoration processes independent of the primary computing path. In onesuch example, the power management controller 1760 is configured to senda signal to effect stoppage of clocks for the processing device 1700 inresponse to the determination that the power quality is below thethreshold. The voltage regulator 1770 can then send a disconnect signalto the power management controller 1760 in response to disconnecting theoutput power supply rail 1755 from the input power supply 1750. Thepower management controller 1760 sends a backup signal to the at leastone non-volatile logic controller 1710 in response to receiving thedisconnect signal. Upon completion of the backup of system state intoNVL arrays, the power can be removed from the SOC, or can continue todegrade without further concern for loss of machine state.

The individual elements that make the determination of power quality canvary in different approaches. For instance, the voltage regulator 1770can be configured to detect the power quality's rising above thethreshold and, in response, to send a good power signal to the powermanagement controller 1760. In response, the power management controller1760 is configured to send a signal to provide power to the plurality ofnon-volatile logic element arrays 1710 and the at least one non-volatilelogic controller 1730 to facilitate restoration of the machine state.The power management controller 1760 is configured to determine thatpower up is complete and, in response, send a signal to effect releaseof clocks for the processing device 1700 wherein the processing device1700 resumes operation from the machine state prior to the determinationthat the power quality was below the threshold.

To assure that the processing device 1700 has enough power to complete abackup process, a charge storage element 1790 is configured to providetemporary power to the processing device 1700 sufficient to power itlong enough to store the machine state in the plurality of non-volatilelogic element arrays 1710 after the output power supply rail 1755 isdisconnected from the input power supply 1750. The charge storageelement 1790 may be at least one dedicated on-die (or off-die) capacitordesigned to store such emergency power. In another approach, the chargestorage element 1790 may be circuitry in which naturally occurringparasitic charge builds up in the die where the dissipation of thecharge from the circuitry to ground provides sufficient power tocomplete a backup operation.

FIG. 18 is a flow chart illustrating operation of powering a processingdevice described above. The method includes operating 1802 a processingdevice using a plurality of volatile storage elements. A primary logiccircuit portion of individual ones of the plurality of volatile storageelements is powered 1804 by a first power domain, and a slave stagecircuit portion of individual ones of the plurality of volatile storageelements is powered 1806 by a second power domain. Data in the pluralityof volatile storage elements is stored 1808 in a plurality ofnon-volatile logic element arrays. After backup is complete, all powerdomains are powered down 1810 to conserve energy. At 1814, during awrite back of data from the plurality of non-volatile logic elementarrays to the plurality of volatile storage elements, the first powerdomain is powered down and the second power domain and third powerdomain are powered up.

So configured, with reference to the example discussed above, during thetime period in which the FF state is being restored from NVLmini-arrays, the bulk of the logic will remain powered down. Thisprevents spurious and repetitive data transitions within thecombinational logic cones served by the groups of FF's during therestoration of the system state. Further power savings can be realizedby powering 1812 the plurality of non-volatile logic element arrays by athird power domain and powering down 1816 the third power domain duringregular operation of the processing device. Accordingly, there is nopower leakage to the NVL arrays during regular operation. Any one of thepower domains can remain powered on during a time indicated above to bepowered down with attendant loss of power savings.

A further example method of powering various portions of a computingdevice as described herein is illustrated in FIG. 19. The methodincludes operating 1902 a processing device using a plurality ofvolatile storage elements and in response to any of a variety ofprompts, transitioning to a backup mode. Examples of the prompts thatcan effect entry into a retention mode include receiving an externalpower request 1906, receiving a software power request 1908, anddetecting a power quality reduction 1910. When transitioning into thebackup mode, a primary logic portion of the volatile storage elements ispowered 1912 by a first power domain that is powered on, a slave stageportion of the volatile storage elements is powered 1914 by a secondpower domain that is powered on, and the non-volatile logic elementarrays are powered 1916 by a third power domain that is powered on. Allthree aspects need power to effect backing up the data from the volatilestorage elements to the NVL arrays. The device then enters a backup mode1907 to store data stored in the plurality of volatile storage elementsin a plurality of non-volatile logic element arrays. After completingthe backup mode, the method includes entering 1918 a sleep mode andpowering down the first power domain, the second power domain, and thethird power domain.

Next, the method includes receiving 1920 an indication of powerrestoration and entering 1922 a system restore mode including poweringon the second power domain and the third power domain and keeping thefirst power domain powered down to avoid parasitic power loss in theprimary logic circuit portion. When transitioning into the restore mode,the volatile storage elements are placed 1923 into a retention mode,which freezes the individual element's random states prior to beingoverwritten with the data from the NVL arrays. Completion of the systemrestore mode is determined 1914. At this point, a clock can be applied1926 to the system clock domain to resume computation of the computingdevice. The computing device can then enter 1928 a regular operationmode including keeping power on the second domain, powering on the firstpower domain, and powering down the third domain. By one approach, powerdomain configuration can be set prior to applying the clock and resumingregular operation mode.

System Example

FIG. 20 is a block diagram of another SoC 2000 that includes NVL arrays,as described above. SoC 2000 features a Cortex-M0 processor core 2002,universal asynchronous receiver/transmitter (UART) 2004 and SPI (serialperipheral interface) 2006 interfaces, and 10 KB ROM 2010, 8 KB SRAM2012, 64 KB (Ferroelectric RAM) FRAM 2014 memory blocks, characteristicof a commercial ultra low power (ULP) microcontroller. The 130 nm FRAMprocess based SoC uses a single 1.5V supply, an 8 MHz system clock and a125 MHz clock for NVL operation. The SoC consumes 75 uA/MHz & 170 uA/MHzwhile running code from SRAM & FRAM respectively. The energy and timecost of backing up and restoring the entire system state of 2537 FFsrequires only 4.72 nJ & 320 ns and 1.34 nJ & 384 ns respectively, whichsets the industry benchmark for this class of device. SoC 2000 providestest capability for each NVL bit, as described in more detail above, andin-situ read signal margin of 550 mV.

SoC 2000 has 2537 FFs and latches served by 10 NVL arrays. A central NVLcontroller controls all the arrays and their communication with FFs, asdescribed in more detail above. The distributed NVL mini-array systemarchitecture helps amortize test feature costs, achieving a SoC areaoverhead of only 3.6% with exceptionally low system level sleep/wakeupenergy cost of 2.2 pJ/0.66 pJ per bit.

Although the invention finds particular application to microcontrollers(MCU) implemented, for example, in a System on a Chip (SoC), it alsofinds application to other forms of processors. A SoC may contain one ormore modules which each include custom designed functional circuitscombined with pre-designed functional circuits provided by a designlibrary.

While the invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various other embodiments of the invention will beapparent to persons skilled in the art upon reference to thisdescription. For example, other portable, or mobile systems such asremote controls, access badges and fobs, smart credit/debit cards andemulators, smart phones, digital assistants, and any other now known orlater developed portable or embedded system may embody NVL arrays asdescribed herein to allow nearly immediate recovery to a full operatingstate from a completely powered down state.

While embodiments of retention latches coupled to a nonvolatile FeCapbitcell are described herein, in another embodiment, a nonvolatile FeCapbitcell from an NVL array may be coupled to flip-flop or latch that doesnot include a low power retention latch. In this case, the system wouldtransition between a full power state, or otherwise reduced power statebased on reduced voltage or clock rate, and a totally off power state,for example. As described above, before turning off the power, the stateof the flipflops and latches would be saved in distributed NVL arrays.When power is restored, the flipflops would be initialized via an inputprovided by the associated NVL array bitcell.

The techniques described in this disclosure may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the software may be executed in one or more processors,such as a microprocessor, application specific integrated circuit(ASIC), field programmable gate array (FPGA), or digital signalprocessor (DSP). The software that executes the techniques may beinitially stored in a computer-readable medium such as compact disc(CD), a diskette, a tape, a file, memory, or any other computer readablestorage device and loaded and executed in the processor. In some cases,the software may also be sold in a computer program product, whichincludes the computer-readable medium and packaging materials for thecomputer-readable medium. In some cases, the software instructions maybe distributed via removable computer readable media (e.g., floppy disk,optical disk, flash memory, USB key), via a transmission path fromcomputer readable media on another digital system, etc.

Certain terms are used throughout the description and the claims torefer to particular system components. As one skilled in the art willappreciate, components in digital systems may be referred to bydifferent names and/or may be combined in ways not shown herein withoutdeparting from the described functionality. This document does notintend to distinguish between components that differ in name but notfunction. In the following discussion and in the claims, the terms“including” and “comprising” are used in an open-ended fashion, and thusshould be interpreted to mean “including, but not limited to . . . .”Also, the term “couple” and derivatives thereof are intended to mean anindirect, direct, optical, and/or wireless electrical connection. Thus,if a first device couples to a second device, that connection may bethrough a direct electrical connection, through an indirect electricalconnection via other devices and connections, through an opticalelectrical connection, and/or through a wireless electrical connection.

Although method steps may be presented and described herein in asequential fashion, one or more of the steps shown and described may beomitted, repeated, performed concurrently, and/or performed in adifferent order than the order shown in the figures and/or describedherein. Accordingly, embodiments of the invention should not beconsidered limited to the specific ordering of steps shown in thefigures and/or described herein.

It is therefore contemplated that the appended claims will cover anysuch modifications of the embodiments as fall within the true scope ofthe invention.

What is claimed is:
 1. A computing device apparatus providingnon-volatile logic based computing, the apparatus comprising: aplurality of non-volatile logic element arrays; a plurality of volatilestorage elements comprising retention flip flop circuits; at least onenon-volatile logic controller configured to control the plurality ofnon-volatile logic element arrays to store a machine state representedby the plurality of volatile storage elements and to read out a storedmachine state from the plurality of non-volatile logic element arrays tothe plurality of volatile storage elements; wherein individual flip flopcircuits comprise a primary logic circuit portion powered by a firstpower domain and a slave stage circuit portion powered by a second powerdomain; wherein the first power domain is configured to be powered downand the second power domain is active during write back of data from theplurality of non-volatile logic element arrays to the plurality ofvolatile storage elements.
 2. The computing device apparatus of claim 1wherein the plurality of non-volatile logic elements are configured tobe powered by a third power domain.
 3. The computing device apparatus ofclaim 2 wherein the third power domain is configured to be powered downduring regular operation of the computing device apparatus.
 4. Thecomputing device apparatus of claim 1 wherein the at least onenon-volatile logic controller is configured to generate a controlsequence for saving the machine state to or retrieving the machine statefrom the plurality of non-volatile logic element arrays.
 5. Thecomputing device apparatus of claim 1 wherein individual ones of theplurality of volatile storage elements comprise: a data input portconfigured to insert data from one of the non-volatile logic elementarrays to an associated volatile storage element by allowing passage ofa stored data related signal from the one of the non-volatile logicelement arrays to a slave stage of the associated volatile storageelement in response to receiving an update signal from the at least onenon-volatile logic controller on a data input enable port to trigger thedata input port.
 6. The computing device apparatus of claim 1 furthercomprising a multiplexer connected and configured to pass states from aplurality of the individual ones of the plurality of volatile storageelements for essentially simultaneous storage in an individual one ofthe plurality of non-volatile logic element arrays.
 7. The computingdevice apparatus of claim 1 wherein the computing device is configuredto operate in a retention mode in which a clock of the computing deviceis suppressed for the volatile storage elements, and the non-volatilelogic controller is configured to control and effect storage of datafrom individual ones of the volatile storage elements into individualones of the non-volatile storage elements.
 8. The computing deviceapparatus of claim 1 wherein the computing device is configured tooperate in a restore mode in which a clock of the computing device issuppressed for the volatile storage elements, and the non-volatile logiccontroller is configured to control and effect transfer of data fromindividual ones of the non-volatile storage elements into individualones of the volatile storage elements.
 9. The computing device apparatusof claim 1 wherein the at least one non-volatile logic controller isconfigured to operate independently from a central processing unit forthe computing device apparatus.
 10. The computing device apparatus ofclaim 1 wherein the at least one non-volatile logic controller isconfigured to control a built-in-self-test mode where all zeros or allones are written to at least a portion of a non-volatile logic elementarray of the plurality of non-volatile element arrays and then it isdetermined whether data read from the at least a portion of anon-volatile logic element is all ones or all zeros.
 11. The computingdevice apparatus of claim 1 wherein individual ones of the plurality ofvolatile storage elements comprise one of a group consisting of: flipflop circuit elements, latches, integrated clock gating cells, andcombinations thereof.
 12. The computing device apparatus of claim 1wherein individual ones of the plurality of non-volatile logic elementarrays comprise one of the group consisting of: ferroelectric capacitorbit cells, flash memory, magnetoresistive random access memory, spintorque magnetoresistive random access memory, and combinations thereof.13. A method comprising: operating a processing device using a pluralityof volatile storage elements; storing data stored in the plurality ofvolatile storage elements in a plurality of non-volatile logic elementarrays; powering a primary logic circuit portion of individual ones ofthe plurality of volatile storage elements by a first power domain;powering a slave stage circuit portion of individual ones of theplurality of volatile storage elements by a second power domain;powering down the first power domain and maintaining power in the secondpower domain during a write back of data from the plurality ofnon-volatile logic element arrays to the plurality of volatile storageelements.
 14. The method of claim 13 further comprising: powering theplurality of non-volatile logic element arrays by a third power domain;powering down the third power domain during regular operation of theprocessing device.
 15. The method of claim 14 wherein the storing datastored in the plurality of volatile storage elements in the plurality ofnon-volatile logic element arrays comprises entering a retention mode tostore the data and powering on the first power domain, the second powerdomain, and the third power domain.
 16. The method of claim 15 furthercomprising after completing the retention mode, entering a sleep modeand powering down the first power domain, the second power domain, andthe third power domain.
 17. The method of claim 16 further comprisingreceiving an indication of power restoration and entering a systemrestore mode including powering on the second power domain and the thirdpower domain and keeping the first power domain powered down.
 18. Themethod of claim 17 further comprising determining completion of thesystem restore mode and entering a regular operation mode includingkeeping power on the second domain, powering on the first power domain,and powering down the third domain.
 19. A computing device apparatusproviding non-volatile logic based computing, the apparatus comprising:at least one non-volatile logic controller separate from a centralprocessing unit for the computing device apparatus; a plurality ofnon-volatile logic element arrays comprising ferroelectric capacitor bitcells powered by a third power domain; a plurality of volatile storageelements comprising retention flip flop circuit, wherein individual onesof the plurality of the flip flop circuits comprise: a primary logiccircuit portion powered by a first power domain, a slave stage circuitportion powered by a second power domain, and a data input portconfigured to insert data from one of the non-volatile logic elementarrays to an associated volatile storage element by allowing passage ofa stored data related signal from the one of the non-volatile logicelement arrays to a slave stage of the associated volatile storageelement in response to receiving an update signal from the at least onenon-volatile logic controller on a data input enable port to trigger thedata input port; wherein the at least one non-volatile logic controlleris configured to control the plurality of non-volatile logic elementarrays to store a machine state represented by the plurality of volatilestorage elements and to read out a stored machine state from theplurality of non-volatile logic element arrays to the plurality ofvolatile storage elements; wherein the first power domain is configuredto be powered down and the second power domain is active during writeback of data from the plurality of non-volatile logic element arrays tothe plurality of volatile storage elements; wherein the third powerdomain is configured to be powered down during regular operation of thecomputing device apparatus.